It is known that in electrical appliances a resonant motor may be provided to drive mechanical components into vibratory motion. The resonant motor may e.g. be realized as an oscillating electric motor. Such resonant motors may be used in electric shavers or electric toothbrushes, in which a working amplitude of a driven mechanical component is created without a gearing mechanism. A mechanical component intended for vibratory motion during operation may be an armature of the resonant motor, a drive shaft, or a component coupled to the drive shaft, such as a brush head for an electric toothbrush.
To achieve good efficiency of the driving of the resonant motor, it is desirable that the driving frequency with which the resonant motor is provided, either (1) coincides with the resonance frequency of the spring-mass system determined by the resonant motor and the driven component or (2) has a certain predetermined distance to the resonance frequency.
It is known that a resonant drive of an electrical appliance can be powered by applying a supply voltage such that a drive current having a fixed frequency that is near the resonance frequency of the spring-mass system builds up. However, the resonance frequency of the relevant spring-mass system depends on several factors that can change while the electric appliance is being operated. In particular, the mechanical load on the driven mechanical component (e.g. a brush head of a toothbrush) may fluctuate, e.g. when the user changes the pressure on the bristles of a toothbrush while brushing teeth.
This may result in a certain change of the actual resonance frequency of the spring-mass system. Since in this case the efficiency and/or the performance of the small electrical appliance may diminish, it would therefore be desirable if the change in the resonance frequency can be detected to improve operation of the electric appliance.
Thus, there is a desire to provide a method and an electrical appliance in which a variation of the actual resonance frequency and/or a mechanical load on the driven vibrating component of the electrical appliance can be detected during operation in an inexpensive and easy to implement manner.